One type of conventional automatic transmission incorporated into vehicles or the like comprises a first planetary gear unit with two rows of linked planetary gearing, and a second planetary gear unit which reduces the rotational speed of the input shaft (for example, see Japanese Unexamined Patent Application Publication No. 4-125345). The transmission disclosed in this publication provides, for example, six forward speeds and one reverse speed by input of reduced speed rotation from a second planetary gear unit via a clutch to, for example, one of four rotary components of the first planetary gear unit. Further, in the case of fourth speed forward, for example, when the rotation of the input shaft is input into two of the rotary components of the first planetary gear unit, this fourth speed forward can be become a directly coupled state, with the output at the same rotational speed as the input shaft.
The above-described automatic transmission comprises two clutches for inputting the rotation of the input shaft into two of the rotary components of the first planetary gear unit, and a second planetary gear unit for outputting the reduced rotation into the rotary components of the first planetary gear unit. However, if those two clutches or the hydraulic servos that control their engagement are positioned between the first and second planetary gear units, the element for transmitting the reduced speed rotation of the second planetary gear unit to a rotary component of the first planetary gear unit must be axially elongated.
The elongation of the element that transmits the reduced speed rotation means that the unit transmitting that rotation with high torque must also be elongated. Further, if an elongated element must withstand the larger torque, it must be fabricated of a relatively thicker material which prevents making a more compact automatic transmission. Further, because the weight of such a member would be increased, not only is the objective of a lightweight automatic transmission defeated, but also inertia (inertial force) would increase, reducing the controllability of the automatic transmission and increasing shock in speed changes.
Further, for example, in order to selectively input the reduced speed rotation output to the first planetary gear unit from the second planetary gear unit, a clutch or brake must be provided. In the case that a clutch is provided, this clutch and the above-described two clutches, a total of three clutches, are necessary. A clutch generally includes a clutch drum that transmits the input rotation to friction plates, and therefore, for example with a problem such as relative rotation, oil pressure to the oil compartment of the hydraulic servo of the clutch must be supplied from the mid-section of the automatic transmission.
However, if the aforementioned three clutches are arranged in series on one axial side of the planetary gear unit, for example, the oil lines for supplying oil pressure to the three hydraulic servos must be provided in triplicate in the mid-section of the automatic transmission, and the configuration of the oil lines become complicated.